We shall continue to investigate the function and source of testosterone production during the second half of pregnancy in the rat and mouse. Maternal ovariectomy, adrenalectomy or fetectomy or fetectomy will be performed in mice; in pregnant rats, similar procedures will be used in addition to cannulation of uterine, ovarian and adrenal veins. The role of placental hormones in the inhibition of pituitary luteotropic function at midpregnancy will be sought in mice caused to implant blastocyts at varying intervals following mating (delayed implantation model). The biochemical basis for altered target organ responsiveness in selected strains of mice will be studied by determining the ability of injected sex steriods and gonadotropins to induce specific receptors and bind specific receptors. Steroid production in ovaries removed from control, rapid growth and large litter size lines of mice will be examined in culture (these lines differ dramatically in the pattern of steroid secretion during pregnancy). In unrelated studies, MSH will be injected into a melanophore and the same cell will be marked by also injecting a fluorescent dye (e.g., procion yellow) into that cell. The technique involves the use of multibarreled pipettes and the subsequent sectioning of frog skin. The use of this precise method of localizing the particular cell into which MSH was injected will substantiate our observations that MSH has no effect on melanosome dispersion when injected intracellularly into frog melanphores.